The present invention relates to an image reading device in which an image sensor mounted on a carriage is moved parallel to and immediately below a transparent board to read the image on a document placed on the transparent board.
Conventionally, an image reading device that performs an image reading for a still document has been well known. The image reading for the still document is performed by moving an image sensor reciprocally along a platen glass on which the document is placed. Such an image reading is known as an image reading device called a flatbed type.
FIG. 1 is a sectional view showing the inner constitution of a conventional image reading device. As shown in FIG. 1, a platen glass 91 is provided so as to be exposed on the upper surface of a housing 90. On the platen glass 91, a document to be read is placed. An image reading unit 92 is provided in the housing 90. The image reading unit 92 has an image sensor 93. The image sensor 93 is a contact image sensor (referred to as the “CIS” hereinafter), The image sensor 93 is mounted on a carriage 94. The carriage 94 is supported by a guide shaft 95 so as to be slid in the axial direction of the guide shaft 95. The carriage 94 is connected to a timing belt of a belt driving mechanism, for example. When the driving force is transmitted from the belt driving mechanism, the carriage 94 is slid in the predetermined direction (the above axial direction). In addition, the belt driving mechanism is omitted in FIG. 1.
Rollers 96 are provided at both ends of the image sensor 93 so as to be projected from the upper surface of the image sensor 93. In addition, a coil spring 97 is provided between the image sensor 93 and the carriage 94. The image sensor 93 is elastically forced toward the platen glass 91 by the coil spring 97 and the rollers 96 are pressed against the platen glass 91. When the carriage 94 is slid, the image sensor 93 mounted on the carriage 94 is also moved. The rollers 96 pressed against the platen glass 91 roll while the image sensor 93 is moved. The image sensor 93 applies light to the document placed on the platen glass 91 while being slid and outputs an electric signal according to its reflected light intensity. The image reading of the document is performed based on the electric signal.
The CIS used as the image sensor 93 has a small focal depth as compared with a reduced optical image reading unit using a CCD(Charge Coupled Device), for example. When the document is positioned out of the focal depth of the image sensor 93, the image sensor 93 cannot focus on the document. As a result the quality of the read image is inferior. Therefore, the distance between the upper surface of the image sensor 93 and the upper surface of the platen glass 91 on which a document is placed is desirably kept within a predetermined range based on the focal distance. According to the above image reading device, the distance from the upper surface of the image sensor 93 to the back surface of the platen glass 91 is set by the rollers 96 projected from the upper surface of the image sensor 93 and pressed against the platen glass 91, so that a desired and constant focal distance is maintained.
The platen glass 91 could be bent by external force. For example, when a book document is pressed against the platen glass 91 at the time of image reading, the weight of the book document and pressing force by a user are applied to the platen glass 91. Although the platen glass 91 is prevented from being bent by increasing its thickness, a demand for reduction in weight and cost cannot be implemented.
For example, when the center of the platen glass 91 is bent downward, the distance from the vicinity of the center to the upper surface of the image sensor 93 is shortened, so that it could deviate from the focal depth of the image sensor 93. As described above, according to the constitution in which the distance between the platen glass 91 and the image sensor 93 is kept constant by the rollers 96 provided at both ends of the image sensor 93, when the platen glass 91 is excessively bent downward, the platen glass 91 could be in contact with the vicinity of the center of the upper surface of the image sensor 93. Especially, when it comes in contact with the platen glass 91 while the image sensor 93 is moved, the image sensor 93 and the platen glass 91 could be damaged. In order to solve the above problem, it is proposed that a roller is further provided in the vicinity of the center of the image sensor 93 with a predetermined space from the platen glass 91. Thus, the bent platen glass 91 comes in contact with the roller at the center and it is prevented from being bent excessively (refer to Japanese Patent Application Laid-Open No. 8-228266 (1996)).